The present invention is generally related to a communication cable and, more particularly, is related to the core and buffer tubes of such cables.
Optical fiber cables have been used for many years to transmit information at high rates and very long distances. The transmission media of the optical fiber cable are hair-thin optical fibers protected from external forces and elements by precisely designed and manufactured cable structures. There are several relatively common cable structure families currently being used to protect these hair-thin optical fibers. Such cable structure families include the loose tube, the central core, and the tube-in-tube structures.
Optical fibers are relatively delicate, compared to previous types of communication media. Typically made of glass, the fibers are not ductile and can be broken or cracked, either of which can destroy or degrade the signal being transmitted. Therefore, optical fibers are housed in rugged cable structures to protect the fibers from mechanical damage during both installation and service. During installation, the cable structure can see substantial compressive and tensile stresses, e.g., while being pulled into ground conduits. After installation, the cable structure can be damaged mechanically by gnawing rodents, or from crushing due to sharp impacts. In addition, the quality of a signal transmitted through an optical fiber is also sensitive to tensile or compressive stresses applied to the cable structure, e.g., those encountered during bends or turns in the cable route, or while suspended between telephone poles. Stresses encountered during service may change significantly as a result of environmental temperature variations, which cause expansion and contraction of cable components.
Accordingly, the components used in each type of cable structure are designed to protect the delicate optical fibers from mechanical and environmental stress. In a loose tube cable, the fibers are contained within multiple rigid, thick-walled extruded buffer tubes that are twisted and bound together. In a central core cable, all of the fibers are contained within a single extruded plastic tube. The tube-in-tube design is a modification of the central-core structure, in which the fibers are contained in flexible thin-walled extruded tubes. Multiple tubes are subsequently placed within a larger extruded tube, similar to that used in central-core cables. In all of these design families, the tube(s) typically also contain a thixotropic, petroleum-based gel to block water penetration and provide some mechanical protection to the fiber.
In all cases, the structure(s) containing the fiber are then sheathed in a continuous, high speed extrusion operation. In each design family, the majority of tensile loads are carried by various types of tensile stiffness members included in the cable, e.g., steel wires, rigid epoxy/glass rods, flexible epoxy/glass rovings, or aramid yards.
However, the mechanism of protection against compressive loads and sharp impacts is different in each cable design family. In the central core structure, resistance to compression is typically provided by the combination of rigid strength members included in the outer plastic sheath, the extruded plastic core tube, the extruded plastic outer jacket, and the thixotropic cable gel. In this design, inexpensive polyolefins are typically used for both the core tube and jacket. The majority of the resistance to compression is typically provided by relatively large rigid strength members, which provide resistance to both tensile and compressive stress. Cables in the tube-in-tube design family utilize sheaths similar or identical to those used for central core cables, with compressive resistance typically derived from rigid strength members within the outer extruded plastic jacket.
In the loose tube cable family, the buffer tubes are generally stranded around a rigid steel or epoxy/glass rod at the core of the cable, and this strength member does not provide resistance to compressive stresses or crushing. As such, the plastic tubes, the thixotropic gel, and the extruded polyolefin sheath must provide the majority of the resistance to compressive stress. Traditionally, the individual tubes have relatively thick walls, and have been fabricated from costly, high-modulus engineering plastics such as poly(butylene terephtalate) and polycarbonate. The thick tube walls can lead to undesirably large cable diameters, making it difficult to install cables in crowded or small cable ducts.
In all of these designs, some resistance to compressive deformation is provided by the thixotropic waterblocking gel contained within the tube or tubes. As these gels are incompressible, they also tend to impart some compression/crush resistance to cables. However, typical cable gels are generally a nuisance during cable installation. Fibers must be completely clean prior to splicing during installation, a process that takes a long time, as gels are typically sticky and hard to remove. Therefore, it is desirable to reduce or eliminate the amount of gel used in future cable designs. One of the benefits of the tube-in-tube design is minimization of gel usage. See for example, U.S. Pat. No. 5,155,789, to Le Noane. et al. and U.S. Pat. No. 5,751,880 to Gaillard. In other cable designs, the gel may be eliminated completely, e.g., U.S. Pat. Nos. 4,909,592 and 5,410,629, both to Arroyo and European Patent No. 0 945 746 A2 to Okada. However, minimization or removal of the cable gel tends to reduce resistance to crush or compressive deformation.
Illustratively, U.S. Pat. No. 5,131,064 to Arroyo, et al., discloses a central core cable having strength rods and a lightning-protective sheath system comprising a thermal barrier, which are disposed between the core of the cable and its plastic jacket. The thermal barrier comprises a textile of glass yarns that have been woven into a unit and then sandwiched between a pair of tapes together with a waterblocking material such as a superabsorbent powder. The glass yarns undulate in the longitudinal direction, not only because of their weaving pattern, but also because the tape follows the undulations of a corrugated metallic shield. Such undulations preclude the tape from receiving any portion of the load until the cable has already been elongated. Because the disclosed tape has a very low tensile strength, 420 Newtons per centimeter of width, the cable tensile strength effectively comes from rigid strength rods that are embedded in the plastic jacket. In addition, the majority of the resistance to compression comes from these rigid rods as well. However, these rods are less flexible than the woven tape, thereby reducing the flexibility of the entire cable. Further, if a pair of rods are used and are positioned diametrically opposite each other on either side of the core, they make the cable inflexible in all but one plane and much more difficult to handle and install.
Another example is U.S. Pat. No. 4,730,894 to Arroyo, which discloses an optical fiber cable that includes a plurality of equally spaced strength members disposed on a carrier tape and held in place by an adhesive. Once a plastic jacket is extruded onto the strength members, they are coupled to the jacket and provide tensile strength to the cable. Typically, these strength members will be rigid epoxy-glass composites, which will provide for both tensile and compressive stiffness. However, increased compressive stiffness correlates to increased flexural stiffness and, therefore, decreased cable flexibility, which makes cables more difficult to handle and to install. To protect the valuable optical fibers, cable flexibility generally has been sacrificed in conventional cables.
Yet a further example, U.S. Pat. No. 5,838,864 to Patel et al., discloses a cable with a dielectric strength member system that attempts to maximize the flexibility of the cable by using a flexible woven strength tape to carry to majority of the tensile loads. To control post-extrusion shrinkage and provide for some resistance to compressive stress, two rigid epoxy-fiber rods are embedded in the jacket, diametrically opposite one another, on either side of the core. However, as these rods do not have to carry tensile load, their size is minimized and, therefore, the overall cost of the strength system is reduced. Further, the volume of jacket material required to encase the smaller strength rods is less than for larger rods, further reducing the cost of the overall cable sheath. Still, the strength system is more expensive and complex than is desirable because of the need for two types of strength systems.
Therefore, there appear to be fundamental deficiencies in the mechanisms used for providing for resistance to compressive deformation and crushing in loose-tube, central-core, and tube-in-tube fiber optic cables. Known central-core and tube-in-tube sheath designs require large rigid strength members which make the cable more expensive, stiff, and difficult to handle. Known loose tube sheath designs require the use of large amounts of expensive engineering plastics, and the relatively large wall thickness of these tubes leads to undesirably large cable diameters.
If the material used for the core tube in central-core or tube-in-tube cables had enhanced mechanical robustness, strength members with reduced compressive stiffness could be employed, increasing cable flexibility and reducing cost. Likewise, if such a material could be utilized for buffer tubes, the thickness of the buffer tubes could be decreased. Furthermore, utilization of such a material would allow for further minimization or elimination of troublesome cable gel. Accordingly, what is sought is a core tube/buffer tube material which would provide increased resistance to crushing and other compressive deformations.
Heretofore, some cables have incorporated materials containing nucleating agents, e.g., inorganic materials, salts of aliphatic monobasic or dibasic acids, or alkali metal or aluminum, salts of aromatic or alicyclic carboxylic acids, and filler materials, e.g., talc, glass fiber, and glass spheres, into buffer or core tubes to give the desired properties of high strength, low shrinkage, good flexibility, improved processibility and low cost. An example of such a cable is described in U.S. Pat. No. 5,574,816, (the ""816 patent) issued to Yang et al. Because the fillers or nucleating agents disclosed in the ""816 patent were not fully effective when added to the core or buffer tubes, there is still a need for buffer tube or core tube materials with enhanced mechanical properties. Furthermore, the ""816 patent only added the fillers to a polyethylene-polypropylene copolymer resin and does not address the use of fillers that can be added to other types of resins.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
The foregoing problems have been overcome by a communication cable including an outer jacket and either a core tube or plurality of buffer tubes disposed around a central strength member, wherein the core tube or buffer tubes include a resin and high aspect ratio fillers occupying a predetermined volume of the core tube or buffer tube so as to impart crush resistance to the cable. In one embodiment of the invention, the fillers are made of smectite clay, and the smectite clay fillers may be either montmorillonite, kaolinite hectorite, synthetic smectite clays or bentonite. Possible examples of the types of resin that may be incorporated into the core or buffer tubes include for example, but are not limited to impact-modified polypropylene, polyethylenes, polybutylene terephthalate, polycarbonate, ethylene-vinyl acetate copolymers, and polyvinyl chloride and thermoplastic elastomers.
In an alternative embodiment of the present invention, the core tube or the buffer tubes of the cable of the present invention may include two layers, an inner layer and an outer layer, wherein the outer layer includes both resin and fillers, and the inner layer includes a resin without fillers.